So even though it’s 2/3 to 1/3, it’s more like 4/5 – 1/5 – so the b7 is just part of more of the chords that are typically used. And “9” on its own does mean “dominant 9th” which the dominant 7th (b7) is part of.
https://www.reddit.com/r/musictheory/comments/rpvc5c/what_is_the_purpose_behind_having_a_b7_in_some/



